It is often desirable to automatically prevent calls to certain telephone numbers and number classes from a given telephone or telephone line. For example, for some business telephones, it may be appropriate to prohibit making long distance calls in order to prevent employees from making personal long distance calls which are not only expensive, but take the employees' time.
Similarly, on a private line, it may be appropriate to automatically defeat 1-900 calls to prevent children from making calls which may be the sort which a parent wishes to prevent because the content may be inappropriate for a child and, further, because 1-900 calls carry a surcharge which can be exorbitant. It may also be desirable, on a given private line (or even a business line), to automatically defeat even a local, non-commercial call if, for example, the number would permit a child to contact a party which a parent does not wish the child to communicate with, either permanently or temporarily.